It is important to have at least a basic understanding
of the mechanics of an exercise you intend to perform or teach,
whether it's a weight training or flexibility exercise. Exercises
require particular positioning of the body in order for target
muscle groups(s) to be exercised optimally.

You can't just put a weight on top of you head, rotate your
head side to side, and call it a neck rotation exercise. Progressive
resistance torque needs to be applied by situating the load against
a force (commonly weights via gravity) so that the intended movement,
or target muscles are challenged throughout a target range of
motion, typically throughout full
range.

For a better understanding this concept,
let's begin with another ridiculous example of someone attempting
to perform a Barbell
Bench Press, but standing upright instead of lying supine.
The pectoralis muscles,
after all, are suited for drawing the arms forward and inward
across the chest (transverse
adduction). The movement is correct. So why would not this
exercise work? The other crucial requirement obviously involves
the alignment of target muscles against the resistive force.
In the case of free weights, that force is gravity pulling downward.
This means if we are to exercises the desired muscle groups in
this movement, we must orientate our body supine on a bench so
our muscles contract under the force of gravity acting upon the
weight.

However, with machines, we can alter the direction of force.
Gravity still acts on the weight but the machine alters direction
of force via use of lever(s) or cable pulley(s). This means on
lever
chest press machine, for example, we would need to be upright
with the hands behind the lever handles to work these same muscles.
In any case, the alignment of target muscles must be positioned
against the resistive forces, whichever direction they happen
to be orientated.

If we want to target the upper chest (clavicular
head) on some sort of barbell press, we would need to know
the action of this muscle and position our body and its appendages
accordingly. In this case, we would lie on an incline bench,
instead of a flat bench. We would however still need to push
the weight upward against gravity as we do in the Barbell
Incline Bench Press. Notice the similarities and differences
when we perform the Cable
Standing Incline Chest Press targeting the same muscle group
but using a specific positioning on an adjustable dual cable
apparatus to alter the direction of force.

Other subtle variations to the body's alignment or its appendages
can make the difference between targeting the muscles or working
an entirely different set of muscles. In other cases with less
than ideal alignment, the target muscle may still be exercised
somewhat, but likely not as nearly as effective as a more ideal
positioning would allow.

Here are some common alignment errors involving some obvious
and not so obvious examples: